mod-script-tags-wordpress.php

<?php

// use the script loader filter to target specific scripts by handle
add_filter( 'script_loader_src', function( $src, $handle ) {
	if ( in_array( $handle, array( 'script-handle' ) )
		add_filter( 'clean_url', 'mod_script_tag', 11, 1 );
	return $src;
}, 10, 2 );

// change the return value to add any attributes you need on the script tag
if ( ! function_exists( 'mod_script_tag' ) ) {
	function mod_script_tag( $src ) {
		remove_filter( 'clean_url', 'mod_script_tag', 11 );
		return "{$src}' async charset='utf-8";
	}
}